Today I will present you at least an interesting idea, if not, even achievable.
It is a system consisting of several components (not too complicated), which will give you the opportunity to fly.
Helium is primarily needed in gaseous form. Secondly, there is a need for a system that is capable of heating and cooling the helium in a flexible tank (for example a textile material) that is sealed in such a way that it can be caught on the back of a person ( like a backpack).

This tank (or bag) is filled with helium and is surrounded by a plastic pipe (basically a flexible tube) that transports hot, cold, hot water as appropriate.

If we want to rise then we increase the temperature of the helium in the bag and if we want to get down, then we lower the temperature. Basically we float in the air, and we get the control exactly in the way we swim in the water and by adjusting the water temperature in the heating / cooling system.
What happens? I do not know!

swimming in air, dreaming of flying, floating in the air
dreaming of swimming in the air like in water. Do it yourself swimming in air floating device

If we increase the temperature of helium it decreases its density, which means that it floats in a gas with a higher density such as air. The higher the temperature of the helium, the greater the difference in density between the two environments and thus we reach the performance of flying (floating).
It is very important to note that helium is flammable, so pay attention to how we regulate the temperature, a balance must be found between the height we want to rise, body weight and helium temperature so that the helium does not reach a too high temperature.
CAREFUL! It’s just a theory, I haven’t tested it and it might not work!